[whatwg/dom] Add declarative Shadow DOM features (#858)

The explainer for this feature is here: https://github.com/mfreed7/declarative-shadow-dom/blob/master/README.md
The issue discussion is here: https://github.com/whatwg/dom/issues/831
There is a corresponding Pull Request for the HTML spec that goes along with this PR.

- [X] At least two implementers are interested (and none opposed):
   * I'm working to get this implemented in Chromium, tracking bug [here](https://crbug.com/1042130).
   * Other implementers have at least not been negative on the [discussion thread](https://github.com/whatwg/dom/issues/831). Not explicitly positive (yet).

- [X] [Tests](https://github.com/web-platform-tests/wpt) are written and can be reviewed and commented upon at:
   * In WPT: https://wpt.fyi/results/shadow-dom/declarative?label=master&label=experimental&aligned&q=declarative
   * More tests are being implemented, but the basics are there.

- [X] [Implementation bugs](https://github.com/whatwg/meta/blob/master/MAINTAINERS.md#handling-pull-requests) are filed:
   * Chrome: https://crbug.com/1042130
   * Firefox: None yet
   * Safari: None yet

You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/dom/pull/858

-- Commit Summary --

  * Added "is declarative shadow root"
  * Added existing shadow root support
  * Add getInnerHTML()
  * Pulled out "attach a shadow root" algorithm

-- File Changes --

    M Makefile (2)
    M dom.bs (82)

-- Patch Links --

https://github.com/whatwg/dom/pull/858.patch
https://github.com/whatwg/dom/pull/858.diff

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